In recent years, liquid crystal display devices are widely used: For example, the liquid crystal display device is used for personal computers, word processors, amusement machines, televisions and the like. The liquid crystal display devices is, however, a holding type display device in which light emitted for display (hereinafter, this light is referred to as a display light ray) changes continuously as time passes, unlike an impulse type display device such as a cathode ray tube in which a display light ray is momentary. Thus, in general, the holding type display device has a slow response time. Therefore, the holing type display device has a problem in that deterioration of an image, for example, a blur in a moving object, occurs especially when it displays a moving image. In order to display a moving image of high image quality, a method for improving a response property has been explored.
One method proposed for improving the response property is to arrange a hold type display device such as a liquid crystal display device to have a pseudo-impulse type display characteristic similar to that of the impulse-type display device. Namely, the method proposes to arrange the hold type display device such that the a display light ray is momentary or intermittent, as in the cathode ray tube.
The Japanese Laid-Open Patent Publication 66918/2003 (Tokukai 2003-66918, published on Mar. 5, 2003) discloses a display device driven in such a way that blanking data is inserted between image data and image data which are for one frame time so that the image data and the blanking data are displayed alternately within one frame time, whereby this liquid crystal display device has an impulse-type display device characteristic. This makes it possible to prevent deterioration of image quality caused by a blur in a moving image and the like, in no need of a large and complex structure (i.e. avoiding a large and complex structure).
To be more specific, the display device disclosed in Tokukai 2003-66918, as illustrated in FIG. 10, includes a circuit 102 for generating scanning data for multiple scanning, a circuit 103 for generating scanning timing for multiple scanning, and a display element array 106. The circuit 102 inserts blanking data between one-frame-time image data (image data for one frame time) supplied from an image signal source 101. The circuit 103 generates timing for driving a gate line.
As illustrated in FIG. 11, a scanning signal generated in the display device is such that a frame time 301 is divided into two periods, namely, a screen image scanning period 302 and a blanking scanning period 303. In other words, during one frame time gate line selection is carried out twice. During the screen image scanning period 302, the scanning signal generated in this display is written in two gate lines at the same time (that is, the scanning signal is supplied to the two gate lines at the same time to control the gate line in accordance with the scanning signal). In other words, the writing is carried out by two line-selection scanning. By the two line-selection scanning, G1 and G2 are selected so that the scanning signal is written into G1 and G2 at the same time; then G3 and G4 are selected so that a next image signal is written into G3 and G4 at the same time. Subsequently, the blanking data is also written into two lines at a time in the same manner by the two line-selection scanning. With this arrangement, image display and blanking display are carried out within one frame time.
In the following the writing with respect to one pixel in a display array in this arrangement is described. As illustrated in FIG. 12, a frame time 401, which is one frame time, is divided into two periods: an screen image writing period 402 (time during which image is written in) and a blanking writing period 403 (time during which blanking data is written in). A video signal is written in during the screen image writing period 402 and the blanking data is written in during a blanking writing period 403. The blanking data is close to a common-level voltage rather than a gray scale voltage for a screen image. The screen image writing period 402 has a selection period, which is indicated by a gate driving waveform 405, meanwhile the blanking writing period 403 also has a selection period, as indicated by the gate driving waveform 405. During the selection period of the screen image writing period 402, the video signal indicated by a source waveform 407 is written in the pixel and transmittance is increased as indicated by an optical response waveform 409. Then, during a selection period of the blanking writing period 403, a clear command signal illustrated by the source waveform 407 is written in the pixel and transmittance is decreased as indicated by the optical response waveform 409.
By using the driving method mentioned above, display as illustrated in FIG. 13(a) is possible. Namely, an original screen image 801 transmitted from the image signal source 101 is compressed to a half in a vertical direction and blanking data is written into the other half by the circuit 102. The screen image thus prepared is written, as illustrated in FIG. 13(b), is written into two lines at the same time in a timing of the two line-selection scanning. In this way, the screen image data and blanking data are displayed within one frame time in such a manner that a screen image response and a black response are repeated. Accordingly, it becomes possible to cause the liquid crystal display device to have a impulse-type display characteristic. This makes it possible to prevent deterioration of image quality resulting from a blur in a moving image.
Tokukai 2003-66918 also discloses a method by which an original screen image is compressed into one quarter and one frame time is divided into four. With this arrangement, a fast-response screen image (which is prepared by using a fast-response filter in order to give a screen image a fast response property: an original image is emphasized in the fast-response screen image) is written in during one quarter of a frame time. During a next one quarter of the frame time, the screen image is written in. And then, during a remaining half of the frame time blanking data is written in. In this way, a much quicker response is attained.
Furthermore, it is also described in Tokukai 2003-66918 that time taken for writing in one line is substantially halved when the same scanning is carried out line by line.
The Japanese Laid-Open Patent Publication 149132/2002 (Tokukai 2002-149132 published on May 24th, 2002) discloses that a clear command is writing in before each sub-frame time, and an image signal is corrected so that the image signal has larger difference from a clear command signal level. This makes it possible to accelerate a response speed of a liquid crystal and to enhance quality of moving image display.
However, the display device disclosed in Tokukai 2003-66918, which enables a response waveform to raise abruptly from a black level by the fast-response screen image, cannot display a correct screen image if the blanking data has not written in completely. To be more specific, corresponding to an applied voltage illustrated by a dotted-line waveform in the upper part of FIG. 14, the display device has an optical response as indicated by a dotted-line waveform illustrated in the lower part of FIG. 14. In FIG. 14, it is supposed that when a voltage is shifted from a voltage level corresponding to an image signal to V0H corresponding to a clear command signal, polarity of the voltage is inverted. (In FIG. 14, voltages corresponding to transmittance Tx are labeled as follows: VxH stands for a voltage at +driving (i.e. the voltage having the positive polarity) and VxL stands for a voltage at −driving (i.e. the voltage having the negative polarity).)
In other words, the display device in which the blanking data is displayed as disclosed in Tokukai 2003-66918 is based on premises that transmittance is in a steady state at T0 during a clear command signal scanning period 33a as illustrated by a solid line after liquid crystal transmittance has become Ta as a result of the voltage VaL corresponding to a video signal of a preceding frame during an image signal scanning period 32a. Accordingly when the voltage VxH corresponding to the present screen image is inputted during an image signal scanning period 32b, a voltage Vx′H is applied during a time in which the video signal is written in, the voltage Vx′H changing the transmittance of the liquid crystal from transmittance T0 to transmittance Tx that corresponds to the video signal Vx, However, in the reality, because the liquid crystal response speed is slow, a transmittance waveform does not reach T0 during the clear command signal scanning period as illustrated by the dotted line (it becomes T0′ that is higher than T0) and the waveform reaches transmittance Tx″ during the image signal scanning period 32b, the transmittance Tx″ being higher than the target transmittance Tx.
Further, in the case mentioned above, even though the voltage V0 of the clear command signal is constant (VoH or VoL is applied as the voltage V0 depending on the polarity inversion), a value of transmittance T0′ of the liquid crystal at the point when writing in a next signal starts varies in various ways depending on the video signal Va of the preceding frame time. Thus, the voltage Vx′ that produces transmittance Tx varies according to the video signal Vx of a preceding frame. Therefore, it is impossible to display a gray scale of the inputted screen image signal by the conventional method by which a constant voltage is given according to the video signal Vx, a correct gray scale and it becomes impossible to carry out moving image display of high image quality.
Again in the liquid crystal display device disclosed in Tokukai 2002-149132, the screen image signal is based on premises that an initial liquid crystal state of a frame time is uniformed by the clear command signal written in. Thus, the liquid crystal display device does not suppose the case in which a desired uniform transmittance is not attained in a pixel due to the slow liquid crystal response speed even if the voltage corresponding to the clear command signal is applied. When the liquid crystal state is not initially in the uniformed state in the way mentioned above, a voltage applied will not be the voltage that produces desired transmittance. As a result, the image accurately representing the original image signal cannot be displayed.